The conch is used throughout the novel as a symbol of order. First, it is used to call the initial assembly when the boys first arrive, and again for the subsequent meetings. Also, the conch is used to keep order during the meetings when the boys decide that only the person holding the conch may talk. We can see how Piggy desires the importance of the conch, especially when Jack and his tribe raid Piggy and Ralph's camp for Piggy's glasses: Piggy thinks they're after the conch, though. Near the end of the book, Piggy dies and the conch is destroyed, symbolically signalling the end of law on the island.
When the boys first arrive on the island, they agree on the importance of the signal fire; however, as the story progresses, we see how Jack and his tribe neglect to recognize the importance of the fire: some boys forget to keep it going, for example. It seems the signal fire might also be a strong symbol for hope. Once the signal fire is forgotten, hope for rescue seems unimportant to most of the boys, and the focus turns to savagery and the hunt for meat.
The boys use Piggy's glasses to start fires on the island, so the glasses can be a sign of intelligence and perhaps even ingenuity. Piggy sees to have many of the most rational, mature ideas, but he's often ignored because of his seemingly poor leadership and communication skills.